Reece and Warbritton ( l ) , using the colchicine technique, reported that mammary glands of rats pregnant during the first lactation exhibited very little mitotic activity between the fifth day of lactation and the second parturition. This indirect evidence indicated that some of the secretory cells present in the first lactation were probably present in the second lactation in rats pregnant during the first lactation. Hence, it was desirable to obtain direct evidence on this problem.The use of tritiated thymidine to label alveolar cells has the advantage that labeled cells could be traced through successive lactations through autoradiography. Although cell numbers can be estimated by DNA content, the DNA procedure can not be used to determine whether or not cells are involuting in instances where hyperplasia is also occurring. However, by using tritiated thymidine to label cells and the DNA procedure to estimate cell numbers, the percentage of "carryover" of alveolar cells from the first lactation to the second lactation can be determined in animals pregnant during the first lactation.Materials and Methods. Virgin female rats of the hooded Norway strain, weighing 160 to 170 g, were mated. They were maintained at a temperature of 72 t 2°F and under 10 hr of artificial light daily. The animals received Purina Laboratory Chow and water ad libitum. Prior to parturition, female rats were either isolated or cohabitated with a sexually mature male rat in a lactating cage. In cohabitated female rats, vaginal smears were examined each morning following parturition for the presence of sperm (day 1 of pregnan-lPaper of the Journal Series, N.J. Agr. Exp. Sta., Rutgers University, The State University, New Brunswick, N J. cy) until day 3 of lactation at which time the male rat was removed and the litter was standardized at 6 pups. T o aid implantation, litters were removed from mother rats at 4:OO p.m. on day 5 of pregnancy and returned at 8: 00 a.m. on day 6 (2).Rats that received tritiated thymidine were injected subcutaneously on the third day of lactation with 0.3 pCi/g of body weight (sp act = 6.7 Ci/mmole; New England Nuclear, Boston, MA), since La Ganga (3) found that mammary proliferation was greatest at this time in lactating hooded Norway rats. The effect of tritiated thymidine on mammary function (litter growth index and nucleic acid content) was determined in rats sacrificed on day 30 of the first lactation and in rats pregnant during the first lactation and sacrificed on day 4 of the second lactation. The percentage of alveolar cells present in the first lactation that was present in the second lactation (carry-over percentage) of rats pregnant during the first lactation was based on two criteria: the mean labeled daughter cell index and the mean total mammary gland DNA content found after injecting tritiated thymidine on day 3 of the first lactation. Rats were sacrificed on day 3 of the first lactation (1 hr after injection), on day 4 of the second lactation (with and without a nonlactating period prior to the...

OBJECTIVE Little information is available regarding doses of ionizing radiation from medical imaging in the growing population of children undergoing therapy for cancer who are at risk of developing second cancers. The purpose of our study was to estimate the potential excess lifetime cancer incidence and mortality associated with thallium bone imging in pediatric patients. MATERIALS AND METHODS We retrospectively reviewed the medical records of pediatric patients treated between August 1991 and December 2003 for newly diagnosed osteosarcoma who underwent 201Tl imaging as part of the treatment protocol. According to age at diagnosis and doses of 201Tl, we estimated the excess cancer incidence and cancer mortality for boys and girls at 5 and 15 years old. RESULTS The study cohort consisted of 73 patients, 32 males (median age at diagnosis, 14.8 years; age range, 8.1–20.1 years) and 41 females (median age at diagnosis, 13.3 years; age range, 6.0–20.7 years). Patients underwent a total of three 201Tl studies with a median dose of 4.4 mCi (162.8 MBq) (range, 2.2–8.4 mCi [81.4–310.8 MBq]) per study. Total median cumulative patient radiation dose for 201Tl studies was 18.6 rem (186 mSv) (range, 8.4–44.2 rem [84–442 mSv]) for males and 21.5 rem (215 mSv) (range, 7.0–43.8 rem [70–438 mSv]) for females. Estimated excess cancer incidence was 6.0 per 100 (male) and 13.0 per 100 (female) if exposed by 5 years of age; 2.0 per 100 (male) and 3.1 per 100 (female) by 15 years of age. Estimated excess cancer mortality was 3.0 per 100 for males and 5.2 per 100 for females at 5 years of age; 1.0 per 100 (male) and 1.4 per 100 (female) exposed at 15 years of age. CONCLUSION Further reduction of doses in younger patients is needed to consider 201Tl a viable option for imaging osteosarcoma.

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